Acoustic logging systems are routinely used in the oil and gas industry to measure formation acoustic properties of earth formation penetrated by a well borehole. These properties include the compressional and shear velocities of the formation, which are subsequently used to determine a variety of formation parameters of interest such as porosity and pore pressure. Additionally, acoustic logging systems are used to produce acoustic images of the borehole from which well conditions and other geological features can be investigated. Other applications of acoustic logging measurements include seismic correlation and rock mechanic determination.
The downhole instrument or borehole “tool” of an acoustic logging system typically comprises one or more sources of acoustic pressure or “transmitters”, and one or more acoustic receivers. The transmitters and receivers are typically spaced axially on the body of the tool. Multiple transmitters and/or receivers can also be disposed at different radial positions around the tool. A portion of the energy emitted by the one or more transmitters propagates through formation material surrounding the borehole, and is subsequently detected by the one or more receivers. Receiver response is then used to determine properties and parameters of interest.
Typical frequencies used for monopole acoustic tools are between 5 to 20 kiloHertz (KHz). It is desirable to have a transmitter that has the highest possible output at the desired frequency. There are many limitations of achieving this in down-hole tools. The more significant limitations are discussed briefly as follows.
The output of an acoustic transmitter is a function of the physical dimensions of the transmitting element such as piezoelectric material. A typical (LWD) tool, which is typically a drill collar, has a wall thickness of less than 3 inches (7.6 centimeters) and an outer diameter of about 7 inches (17.8 centimeters). If the transmitter is disposed within the wall of the tool, less than half of the wall thickness and a maximum of 2 inches (5.0 centimeters) of the perimeter of the collar can be due to structural restrictions. These restrictions set the maximum dimensions of a transmitter that can be used in an LWD tool.
The frequency of a transmitter comprising a piezoelectric crystal is a function of the physical dimensions of the transmitter. The size required to achieve the desired frequency determines the dimensions of a transmitter and hence limits its output.
Space required to fasten, seal, and mechanically and electrically isolate a transmitter in logging tool of any type adds additional limiting factors to the transmitter dimensions and therefore to the transmitter outputs. In addition, the transmitter must be covered to protect it from mechanical damage during drilling operations that include handling, drilling and tripping of the drill string.
In summary, a transmitter comprising one or more piezoelectric crystal elements, or a “piezoelectric transmitter”, must be dimensioned and geometrically configured to operate within a tool in harsh borehole conditions. The structure required to operationally dispose the transmitter within the tool (such as a drill collar) imposes additional transmitter dimensional restrictions that, in turn, affect energy and frequency output of the transmitter. There is, therefore, a need for a monopole transmitter with optimized acoustic pressure output, with output frequency optimized to fall within a desired frequency range, and with a physical configuration suitable to meet structural restrictions of LWD and MWD logging systems.